Among plasma generated using an electromagnetic wave, microwave plasma is generated by introducing a microwave into a processing chamber, which is depressurized, through a dielectric plate. In a microwave plasma processing apparatus, if the electron density ne of plasma is higher than cut-off density nc, a microwave cannot be introduced into the plasma and thus propagate between a dielectric plate and the plasma. Part of the microwave propagating between the dielectric plate and the plasma is absorbed as an evanescent wave into the plasma to sustain the plasma. The microwave propagating between the dielectric plate and the plasma is called a surface wave.
Propagation modes of the excited surface wave are determined by the frequency of the microwave, the electron density of the plasma, the shape or size of the dielectric plate, the permittivity of the dielectric plate, and the like. In general, a microwave having a frequency of 2.45 GHz is used as excitation means to generate the plasma, and the surface wave generated beneath the dielectric plate from the microwave is a superposition of multiple propagation modes. In particular, a plurality of propagation modes concentrate on an area having an electron density of about 1×1011 cm−3 to 1×1012 cm−3 which is very suitable for processing the target object.
Meanwhile, the propagation modes of the surface wave are discrete with respect to the electron density of the plasma. Accordingly, there is a possibility that the plasma generated using the surface wave that is the superposition of the multiple propagation modes is non-uniform, thereby making it difficult to process the target object.
In order to overcome this problem, a technology of controlling the propagation modes by arranging a conductive plate provided with a plurality of linear projections close to a surface of the dielectric plate facing a substrate is disclosed in Patent Document 1. In the Patent Document 1, when a microwave is introduced into a processing chamber, the microwave selectively passes through slots while avoiding the linear projections of the conductive plate. Mode selection is carried out while the microwave passes through the slots, thereby improving plasma uniformity.
[Patent Document 1] Japanese Laid-Open Patent Publication No. 2000-273646